My son's forehead vein pops out all the time, why is this? How do you fix this without surgery? (Photo)

He has a vein that has popped out for almost a year, he is very active, drinks a ton of water, his blood pressure is normal. But he has acne so he uses washes that dry out his face but he uses a moisturizer. The vein first came after he lost a lot of weight and when puberty started. Please tell us what he is doing wrong and how to fix it since it does bother him a lot.